Recently, increasing demands have been made with respect to the shape and thickness of rolled metal strips (hereinafter referred to as "rolled strip"). Consequently, in order to control a crown of the rolled strip, several methods have been adopted in which a metal strip (hereinafter referred to as "strip") is rolled by upper and lower work rolls which cross in a plane parallel to the strip (hereinafter referred to as "work cross rolling") or is rolled by a pair of work roll and backup roll which cross in the plane (hereinafter referred to as "pair-cross rolling"). These cross rolling methods have already been widely introduced in hot rolling since they exhibit excellent control ability. However, if they are applied to cold rolling which requires high dimensional accuracy, it is necessary to prevent the rolled strip from twisting. In order to respond to demands relating to a wide variety of strip, thicknesses, and widths, a cold rolling mill with a high level of control is required. Although the cross rolling method is excellent with respect to its control ability, it has not been satisfactorily used in cold rolling due to the problem of torsion.
The reason why torsion is caused in the rolled strip by the cross rolling will be explained below by referring to FIG. 4. FIG. 4(a) is an explanatory view illustrating a torsion in the rolled strip 4 when the strip 3 is rolled by a pair of upper and lower rolls 1 and 2 which are crossed in a plane parallel to the strip 3. The rolled strip 4 is pushed down at an edge side A and pushed up at an edge side B during rolling by virtue of the position of the work rolls 1 and 2. This causes torsion in the rolled strip 4.
FIG. 4(b) is an explanatory view illustrating a torsion in the rolled strip 4 when the strip 3 is rolled by cross rolling method. As shown in FIG. 4(b), the rolled strip 4 is subjected to a shearing force F.sub.1 in the strip width direction on the top side and to a shearing force F.sub.2 in the strip width direction on the bottom side due to the fact that rotary axes of the work rolls 1 and 2 are not perpendicular to a rolling direction X. This causes torsion in the rolled strip 4. These torsions occur in opposing directions to each other, but do not cancel each other out since the torsion caused by shearing forces is larger than the torsion caused by the geometry of the work rolls in cold rolling. Consequently, torsion remains; in the rolled strip 4.
In hot rolling, recrystallization takes place in the rolled strip immediately after rolling even if a shearing force which causes torsion acts on the rolled strip. Accordingly, if tension is applied to the rolled strip except at the top and bottom ends thereof and the rolled strip is formed to be virtually flat, residual stress which would cause torsion is disappear. Consequently, there is substantially no problem of torsion.
In cold rolling, there is no stress relief caused by the recrystallization immediately after rolling, and therefore torsion is caused in the rolled strip. In particular, in the case of products without annealing, there is a serious problem with respect to quality, if torsion remains in the rolled strip. Also, the rolled material exhibiting torsion caused a deterioration in strip-passing operations in a process line such as in a continuous annealing line and the like.
Japanese Patent Publication No. 59-41804 (1984) discloses a rolling line in which intersection angles of the upper and lower work rolls are set to be reversed alternately in the order of mills in a rolling line in which cross mills are arranged in tandem. Also, Japanese Patent Public Disclosure No. 59-144503 (1984) discloses a rolling line which has mills with one of a pair of work rolls being arranged in a direction perpendicular to the rolling direction and the other of the rolls being arranged in a direction crossing with the one in a plane parallel to the one of rolls and reverses the arrangement of the work rolls in each mill.
However, even if cold rolling is carried out in these rolling lines, the directions of torsion in the rolled strip are reversed at each rolling stand and torsion remains in the rolled strip after the final pass. Accordingly, the problem of torsion in the rolled strip is not removed.
There is a problem of producing torsion in the rolled strip, as described above, in cross rolling for controlling a strip crown. On the other hand, there are increasing demands for high accuracy of thickness and shape of rolled strips. In the rolling methods disclosed in JPP Nos. 59-41804 and 59-144503, torsion still remains in the rolled strip after a final pass.
On the other hand, in order to produce a metal strip, for example, a cold rolled stainless steel strip, a hot rolled stainless steel strip which is either pickled or annealed and pickled is cold rolled during supply of a lubrication oil thereto, is either annealed and pickled or bright annealed, and the finished by skinpass rolling.
A cold rolled stainless steel strip is required, in particular, to have excellent surface gloss, since it is generally used as a product as finished skinpass rolled. As a method for improving the surface gloss of a cold rolled stainless steel strip, for example, Japanese Patent Public Disclosure No. 2-169108 (1990) discloses a method which anneals and pickles a hot rolled stainless steel strip and then cold rolls the strip at a rolling reduction rate of over 5% by employing work rolls having crossed grooves provided on the surface. This method is intended to flow a lubricant oil existing between the work rolls and the steel strip out of a roll bite by cold rolling the strip by means of work rolls having such crossed grooves provided on the outer surface. If a quantity of the lubricant oil exists in recesses on the surface of the strip between the work rolls and the strip, there is nowhere for the lubrication oil to escape to when the strip contacts with the work rolls in the roll bite. Thus trapped lubrication oil causes surface roughness on the strip prior to being cold rolled leave on the strip after being cold rolled, even if the recesses on the strip after being rolled are made smaller than those prior to being cold rolled.
However, there are the following problems in the method disclosed in JPPD No. 2-189108:
a. it is troublesome to Form cross grooves on the roll; PA1 b. in the case that the grooves are shallow and few, convex-shape defects are formed on the surface of the rolled material and a high pressure acts on the raised portions during subsequent cold rolling, thereby resulting in seizure due to a lack of lubrication; and PA1 c. the crossed grooves are loaded or worn during rolling, thereby reducing an oil lubrication effect.
Amount of the cold rolled stainless steel strip as finished skinpass rolled increase and in particular the excellent surface gloss of the strip has been required. In order to improve the surface gloss of a cold rolled stainless steel strip, it is necessary to reduce a surface roughness of the strip. It is known to reduce a surface roughness of a hot rolled strip before cold rolling or to reduce the surface roughness of the strip during initial passes in cold rolling.
However, it is difficult to realize the method disclosed in JPPD No. 2-169108 and it is also difficult to provide a sufficient surface gloss.
The inventors have discovered that a metal strip surface having a high gloss can be obtained by providing the strip with a sliding force in a strip width direction between the surface of the strip and work rolls so that a surface layer of the metal is deformed by shearing in the strip width direction. The strip is then brought into contact with the work rolls in order to bring about an improvement in its surface gloss. In particular, cross rolling is effective as a method of providing a strip with a sliding force in the strip width direction between the surface of the metal strip and the work rolls.
If the cross rolling mill is applied to all stands in a tandem mill line, however, torsion occurs in rolled strip. These products sometimes are rejected because of the torsion.
An object of the present invention is to provide a method and a mill array of producing a metal strip having no torsion occurring in a rolled strip by means of a relatively simple process upon cold rolling in a cross rolling manner and an excellent surface gloss.
Still another object is to eliminate torsion in a rolled metal strip and to carry out cross type tandem rolling without any deterioration in surface gloss.